how many quotes?

I need a function to fetch the nextval of a sequence (it should do other
things, but my problem is the nextval() call), like this:

CREATE OR REPLACE FUNCTION test() RETURNS INTEGER AS '
DECLARE
val INTEGER;
BEGIN
val:=SELECT nextval(''''''''''usuarios_seq'''''''''');

--do other things
END;
' LANGUAGE 'plpgsql';

SELECT test();
ERROR: syntax error at or near "SELECT" at character 8
CONTEXT: PL/pgSQL function "test" line 4 at assignment

I can't figure out how many quotes I should use to call nextval. I've
read an article at the techdocs about this, but didn't help me. If
anyone knows the *magic* number, please tell me :)

thanks in advance.